Asymptotic Giant Branch (AGB) stars are the main contributors of solid material (dust) to the interstellar medium. The dust forms as gas escaping from the star cools and condenses to form a circumstellar dust shell. Around oxygen-rich stars, most of the dust particles are silicates, but important questions remain about which minerals are formed and whether the grains are crystalline or amorphous (glassy). Dust grains that form below the glass transition temperature (Tg) of a particular mineral should be amorphous, while those that form much above Tg should be crystalline. The most refractory silicates predicted to form are members of the Melilite group of minerals, whose end-members are Gehlenite (Ca2Al2SiO7), Sodium Melilite (CaNaAlSi2O7) and Åkermanite (Ca2MgSiO7). Synthetic glasses corresponding to these end-member compositions were synthesized from oxide and carbonate powders by melting in Pt crucibles in a muffle furnace at temperatures up to 1650˚C. The viscosity of the melts was measured by the parallel-plate technique in the temperature range 655 to 900˚C, over the viscosity range 2x107 to 91012 Pa s. The viscosity-temperature data were interpolated to determine Tg for each glass, taken to be the temperature at which the viscosity is 1012 Pa s. Two naturally occurring mineral samples, of åkermanite and gehlenite, were also melted. At higher temperatures and lower viscosities than about 108 Pa s, crystallization is expected to be rapid relative to the timescale of cooling in circumstellar dust shells. Our experimentally determined glass transition temperatures therefore provide constraints on the nature (crystalline vs amorphous) of several refractory astrominerals predicted to be found in these shells. Future studies of these glasses will include infrared spectroscopy, in order to better interpret astronomical spectra and test for the presence of amorphous dust with these compositions. The results will be used to test competing models for circumstellar dust formation.

URI:

http://hdl.handle.net/10355/2117

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